Heterogeneous cobalt-bonded tungsten carbide

ABSTRACT

Strong, hard, impact-resistant bodies comprising tungsten carbide bonded with from 3 to 25% by weight of heterogeneous cobalt-tungsten solid-solution alloy, useful as cutting tools, are prepared by heating an intimately mixed cobalt/tungsten carbide powder to a temperature above 1000* C and consolidating the powder to a density of at least 98% of its theoretical density, having either. 1. MIXED A CARBON-RICH AND A CARBON-DEFICIENT POWDER TOGETHER PRIOR TO CONSOLIDATION TO PRODUCE A NON-HOMOGENEOUS BINDER; 2. ADDED FREE CARBON TO A CARBON-DEFICIENT POWDER TO PRODUCE LOCAL AREAS WHERE TUNGSTEN IS REMOVED FROM THE BINDER ALLOY AS TUNGSTEN CARBIDE; OR 3. ALLOWED A PORTION OF THE CARBON IN THE TUNGSTEN CARBIDE TO OXIDIZE DURING CONSOLIDATION TO PRODUCE AREAS IN THE BINDER PHASE WHICH ARE THEN CARBON DEFICIENT AND HIGH IN TUNGSTEN.

United States Patent [151 3,660,050 Iler et al. 51 May 2, 1972 54]HETEROGENEOUS COBALT-BONDED 5? ABSTRACT TUNGSTEN CARBIDE [72] Inventors:Ralph K. Iler; Eugene G. Rigby, both of Wilmington, Del.

[73] Assignee: E. I. du Pont de Nemours and Company,

Wilmington, Del.

[22] Filed: June 23, 1969 [21] Appl. No.: 835,817

[52] US. Cl ..29/182.8, 75/204 [51 Int. Cl ..C22c 29/00 [58] Field ofSearch ..29/182.8; 75/204 [56] References Cited UNITED STATES PATENTS3,165,822 1/1965 Beeghly ...29/182.7 3,451,791 6/1969 Meadows ..29/182.8

Primary Examiner-Carl D. Quarforth Assistant ExaminerR. L. TateAttorney-John R. Powell Strong, hard, impact-resistant bodies comprisingtungsten carbide bonded with from 3 to 25% by weight of heterogeneouscobalt-tungsten solid-solution alloy, useful as cutting tools, areprepared by heating an intimately mixed cobalt/tungsten carbide powderto a temperature above 1000 C and consolidating the powder to a densityof at least 98% of its theoretical density, having either.

1. mixed a carbon-rich and a carbon-deficient powder together prior toconsolidation to produce a non-homogeneous binder;

2. added free carbon to a carbon-deficient powder to produce local areaswhere tungsten is removed from the binder alloy as tungsten carbide; or

3. allowed a portion of the carbon in the tungsten carbide to oxidizeduring consolidation to produce areas in the binder phase which are thencarbon deficient and high in tungsten.

8 Claims, 4 Drawing Figures PATENTEDMAY 2 I972 SHEET 10F 4 s R 0 MR E muK H P L A R EUGENE B RIGBY 21 Haw 520 ATTORNEY PATENTEDMAY 2 I972 SHEET2 OF 4 INVENTORS RALPH K. ILER EUGENE B. RIGBY ANGLE f Aim PATENTEBMAY2I912 3 660 050 PEAK POSITION DEGREES 29 INVENTORS RALPH K. ILER EUGENEB. RIGBY ATTORNEY PATENTEDmz 1972 3. 660,050

PEAK POSITION, DEGREES 26 INVENTORS RALPH K. ILER EUGENE B. RIGBYIIETEROGENEOUS COBALT-BONDED TUNGSTEN CARBIDE BACKGROUND OF THEINVENTION This invention relates to hard metal compositions of tungstencarbide bonded with a heterogeneous cobalt-tungsten alloy, to methodsfor preparing them, and to the use of the final products in cutting orshaping very hard materials.

The products of this invention will ordinarily be referred to herein ascobalt-bonded tungsten carbide, a term commonly employed to describe awell-known class of compositions, but it will be understood that thecobalt binder phase contains appreciable amounts of tungsten and is thusin reality a cobalttungsten alloy.

As shown in copending application Ser. No. 660,986, filed Aug. 16, 1967,now U.S. Pat. 3,451,791 substantially nonporous compositions of tungstencarbide bonded with a cobalt alloy having a novel combination ofstrength and hardness are obtained when the cobalt phase contains overabout 8 percent by weight of tungsten in solid solution, the grain sizeof the tungsten carbide is less than a micron and the composition ishomogeneous in composition and structure.

We have discovered a further class of useful carbide structures in whichthe composition of the cobalt alloy binder is not homogeneous but variesfrom region to region on a microscopic scale throughout the composition.In this new class of carbide structures, regions bonded with cobaltsolid-solution alloys rich in tungsten, with high strength and hardnessbut greater brittleness, are interspersed on a microscopic scale withregions bonded with a weaker but more ductile and tougher cobalt phasecontaining less tungsten than in the aforesaid regions. In this wayregions of high strength, modulus and hardness are interdispersed withregions of high duetility and toughness.

The effect of tungsten in solid solution in the cobalt binder, on theacid resistance of the cobalt phase is well known in the art, whichteaches that for at least some tungsten to be free to dissolve in thecobalt phase, the atomic ratio of carbon-totungsten in the system mustbe less than 1.0. However, a carbon-deficiency is generally consideredto be undesirable because during sintering it promotes at least apartial reaction of the tungsten-containing cobalt binder phase withtungsten carbide to form the brittle eta phase, Co .,W C, with attendantloss of desirable strength properties, especially resistance to impact.

Thus, it has been demonstrated by Kubota, Ishida and Ham in the IndianInstitute of Metals Transactions Vol. 9, 132-138, September, 1964, thatin carbon-deficient, cobalt-bonded, tungsten carbide compositions, thehigher the concentration of tungsten in solid solution in the cobalt thegreater the resistance of the metal phase to attack by concentratedhydrochloric acid.

The relationship between acid resistance and the amount of tungsten inthe cobalt metal phase calculated from the data of the above authors forcompositions containing 5% and 25% of cobalt is shown in their FIG. 1.Such behavior is characteristic of a body in which the metal binderphase is homogeneous, the amount of tungsten in the cobalt phase beingrelatively uniform throughout the body.

As shown by these authors, the yield strength of cobalttungsten alloysincreases with tungsten content. However, Adkins, Williams and Jafieeshow these alloys also become more brittle in Cobalt" 1960) page 8, and16-29.

Kubota and associates show that in metal-bonded tungsten carbidecompositions, when the tungsten carbide average particle size is lessthan 3 microns, those bodies which are carbon deficient are inferior instrength. The carbon deficiency, of course, produces a tungsten-richcobalt binder phase. Such fine-grained bodies are reported to be weakerthan bodies of similar fine-grain size which are not carbon-deficientand thus have less tungsten in the cobalt phase.

Regardless of grain size of the tungsten carbide, bodies with moretungsten dissolved in the cobalt are known to have more acid resistance.

We have now found it possible to prepare cobalt-bonded tungsten carbidebodies in which the cobalt binder contains on the average more than 8%by weight of tungsten dissolved in the cobalt, yet in which the cobaltphase is low in acid resistance. The variations in the concentration oftungsten in solid solution in the cobalt binder result in lower averageresistance of the binder phase to removal by hydro-chloric acid, thanwhen the same amount of tungsten is uniformly distributed in the cobaltphase. The products of this invention are thus characterized by having arelatively lowresistance to attack by acid in spite of having asubstantial amount of tungsten dissolved in the cobalt phase.

It is also well-known in the art, that in cobalt-bonded tungsten carbidecompositions, carbon deficiency leads to formation of eta phase, Co W C,during consolidation at high temperature. Formation of eta phase leavesless ductile cobalt binder phase and thus causes brittleness in theresulting bodies. The localized carbon deficiency in the compositions ofthis invention surprisingly does not result in formation of brittlecobalt-bonded compositions. On the contrary, the compositions of thisinvention are surprisingly'tough, frequently possessing transverserupture strengths in excess of commercially available cobalt-bondedtungsten carbide compositions which are free of eta phase.

The dense, cobalt-bonded compositions of this invention are prepared byhot-pressing heterogeneous powder mixtures of cobalt/tungsten carbides.Variations necessary for heterogeneity are induced in the powders by anyone of the following techniques:

Blending dissimilar powders. Variations-can be achieved by selecting andblending dissimilar tungsten carbide powders, each respectively havingatomic ratios of carbon to tungsten of greater and less than one. Evenafter ballmilling the tungsten carbide with cobalt and consolidating todense bodies, local variations in the amount of tungsten dissolves incobalt occur on a microscopic scale, as a result of admixing thedifferent powders.

Blending carbon with powder. Variations can also be achieved by admixingand dispersing small amounts of finely divided carbon in acobalt/tungsten carbide mixture which has a carbon:tungsten atomic ratioless than one. When the composition is heated during consolidation, thecarbon particles dissolve and carburize the region around each particle,raising the local carbon:tungsten ratio to one or higher.

Oxidizing the powder. Variations can also be obtained by slightlyoxidizing cobalt/tungsten carbide powders having a carbon:tungsten ratioof 1.0 or slightly higher. Thus, finely milled powder, when dried, willabsorb from 0.1 to 1.0 percent by weight of oxygen when exposed to air.Oxidation of powder is generally non-uniform, the outer surfaces ofgranules being oxidized first (in a mass, generally the upper surface ofthe powder is oxidized more than the interior).

SUMMARY OF THE INVENTION In summary, this invention is directed to densebodies consisting essentially of tungsten carbide bonded with from 3 to25% by weight of a heterogeneous cobalt-tungsten alloy, said alloyconsisting essentially of cobalt and an average of from 5 to 25% byweight of tungsten, said alloy comprising regions containing less than8% by weight of tungsten interspersedv with regions containing more than8% by weight of tungsten. This invention is further directed to the useof the dense bodies as cutting tools, and to a method for preparingthese compositions comprising milling finely divided carbon with cobaltand tungsten carbide in sufficient amounts to produce a mixturecontaining from 0.01 to 0.5% free carbon and having a carbon:tungstenratio about 1; heating the milled mixture in an inert atmosphere at anelevated temperature; densifying the composition to at least 98% of itstheoretical density by hot-pressing; and then rapidly cooling the densecomposition.

The dense bodies of this invention demonstrate an unusual combination ofextremely good strength and hardness while sacrificing little in suchproperties as ductility and toughness.

As a result, the bodies are useful in a variety of applications ascutting tools and bits with particular advantage in uses usuallyconfined to high speed steel cutting tools.

The overall or average atomic ratio of carbonztungsten can range from0.85 to 1.02, depending on the cobalt content. However, the dense bodiesof this invention have a surprising combination of strength andtoughness in view of prior art teachings, and are exceptionallyeffective for use as tips or bits in metal cutting and metal-removaloperations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts X-ray diffractometertracings of twelve various cobalt-bonded tungsten carbide compositions,indicating the degree of heterogeneity of tungsten in the cobalt binder,along with two reference curves for sodium chloride.

FIG. 2 depicts two X-ray diffraction curves for sodium chloridesuperimposed at the extreme ends of a diffraction curve for cobalt.

FIG. 3 is a plot of the profiles of 14 cobalt-bonded tungsten carbidecompositions analyzed according to procedure B described hereinafter.

FIG. 4 is a plot of the profiles of FIG. 3 after adjusting the relativepositions of the sample patterns by the estimated error in determiningthe sodium chloride peak position.

DESCRIPTION OF THE INVENTION As stated above, this invention is directedto dense bodies of tungsten carbide bonded with a cobalt-tungsten alloy.The bodies have a density of at least 98% of their theoretical densityand comprise a polycrystalline, three-dimensional network structure oftungsten carbide, the pores of which are interpenetrating and filledwith from 3 to 25% by weight ofa threedimensional, continuous,cobalt-tungsten alloy binder phase containing an average of from about 5to 25% and preferably 8 to 12% by weight of tungsten in solid solutionin the cobalt. The cobalt-tungsten binder phase is characterized bybeing heterogeneous and substantially less resistant to dissolution inconcentrated hydrochloric acid at room temperature than the cobalt phasein bodies of similar constitution in which the tungsten is homogeneouslyand uniformly distributed throughout the cobalt phase. The dense bodiesof this invention are prepared by several means including by suitablyadmixing separately prepared tungsten carbide-cobalt powders andconsolidating the powders to density to produce bodies consisting ofinterspersed regions smaller than about 100 microns in cross-section,having substantially different concentrations of tungsten in cobalt,there being present cobalt regions containing more than 8% by weight oftungsten as well as cobalt regions containing less than 8% by weight oftungsten as indicated by the cobalt lattice constants as determined byX-ray diffraction, the difference between the varying regions ordinarilyexceeding 1 to 2% and preferably being at least 2 to 3% or more.

1. DENSE COMPOSITIONS OF THIS INVENTION a. Structure The bodies of thisinvention consist of two interpenetrating continuous phases, the majorone is tungsten carbide and the minor one is cobalt-tungsten alloy. Thelatter is also referred to as a binder phase because it has generallybeen thought that it surrounded and bound together the grains oftungsten carbide. Since it greatly contributes to the strength of thecomposition, it must, in fact, bind the structure together. We havefound additional proof of this by accurately measuring the length of athin bar of a tungsten carbide body of this invention, containingpercent by weight of cobalt-tungsten alloy, and then removing thetungsten carbide phase without disturbing the metal phase, which isporous but coherent, and measuring the length, of this metallicskeleton. We found that it is about two percent shorter than theoriginal length, showing that in the original composition, the metalphase was subjected to a two percent elongation. This shows that thecobalt in bodies of this invention is under considerable tension andstrain, and that it thus keeps the tungsten carbide phase undercompression and truly acts as a binder."

The amount of cobalt metal present as binder in the bonded compositionsof this invention, ranges from about 3 to 25% by weight, preferablyfrom-5 to 12% by weight. Bodies containing an amount of cobalt withinthe 3 to 25% range have a very desirable combination of strength,hardness and toughness, and those bodies containing from 5 to 12% byweight are particularly suitable, because of their toughness, forreplacing high speed steel tools.

The tungsten carbide phase, also referred to as the tungsten carbideskeleton, contributes markedly to the outstanding properties of thedense bodies of this invention. The tungsten carbide skeleton ispolycrystalline; that is, it consists of many small crystals separatedby grain boundaries. Some of these boundaries are scarcely visible whena polished section is etched with acid, which removes cobalt, but can berevealed by etching with a suitable reagent for dissolving tungstencarbide by methods known to those skilled in the art. By these means theindividual grains making up the carbide skeleton can be distinguishedthrough an optical microscope and sur' face replicas can be made andexamined by the electron microscope.

A significant characteristic of the tungsten carbide in the preferredbodies of this invention is the presence of a substantial proportion ofthe structure as a fine grain structure. The carbide grains, as measuredin metallographic polysections described hereinafter, consist of asubstantial proportion having an average grain diameter of less than amicron, but the remainder may be larger than 1 micron.

The tungsten carbide skeleton contributes substantially to the overallstrength and hardness of the dense compositions of this invention.

Another structural characteristic of the compositions of this inventionis the heterogeneous nature of the cobalt-tungsten alloy binder. Thus,where compositions of the prior art are generally characterized ascontaining a binder phase which is essentially homogeneous throughoutthe composition the products of this invention are characterized by avariety of cobalt-tungsten ratios throughout the binder phase.

b. Tungsten in the cobalt In prior art compositions the cobalt phasecontains an amount of tungsten that is related to the atomic ratio ofcarbonztungsten in the body. The tungsten which is not combined withcarbon as tungsten monocarbide, WC, could be present in one of thepossible states which have been described in the prior art incarbon-tungsten-cobalt ternary systems, namely: ditungsten carbide W C;various cobalt tungsten carbide phases such as kappa or eta (Co W C),this latter also being known in some countries as delta"; metallictungsten; the intermetallic compound Co W or in solid solution in thefacecentered cubic form of cobalt which is the main constituent of thebinder phase.

Regardless of the heterogeneous distribution of tungsten in the cobaltphase, in bodies of this invention, it is preferred to have most of thetungsten which is present in the bodies and which is not present astungsten monocarbide, in solid solution in cobalt. By suitably relatingthe atomic ratio of carbon: tungsten to the cobalt content, maintainingthe tungsten carbide with a very fine grain size, permitting at leastsome of the tungsten to dissolve in the cobalt phase before hot pressingand then pressing and cooling rapidly, we have found that it is possibleto maintain a large portion of the tungsten in solution in the cobaltand to minimize formation of eta and other solid phases. By controllingthe conditions during preparation of these compositions, we have foundit possible to vary the amount of tungsten in the cobalt from region toregion throughout the composition. These regions can be ascertained andcharacterized by a variety of techniques. The regions in which thetungsten concentration in the cobalt is less than 8% by weight and thebinder phase is readily attacked by acid, may be as large as microns, orthey may be very small,

such as less than a micron in cross-sectional diameter. Where they arelarge, as when powders in granular form, high and low in carbon aremechanically mixed and hot pressed, the regions can be detected easilyby metallographic procedures. The low carbon regions often contain someeta phase which is easily distinguished. In such cases, the attack ofthe binder by acid occurs irregularly and can be observed in microscopiccrosssection. On the other hand, when the low-tungsten-in-cobalt regionsare only a micron or so in size they can be detected by X-raydiffraction, as will be further described.

0. Carbonztungsten ratio We have found that if the tungsten content ofthe cobalt binder phase exceeds about a third of the metal binder phaseby weight, it becomes very difficult to prevent the conversion ofsubstantial amounts of the cobalt binder to the more brittle eta phase.For this reason, the atomic ratio of carbon: tungsten ordinarily rangesfrom 0.85 up to 1.02 and should be greater than about [1.0-0.0062(Pl)]and less than about [1.0 0.00166 (P-l 5 where P is percent by weight ofcobalt in the composition. A preferred lower limit is about [1.0-0.004(P1)AH( OR A MINIMUM C:W ratio of about 0.90.

On the other hand, the carbon deficiency must be sufficient to provide ameasurable amount of tungsten in the cobalt phase, and the deficiencymust be greater as the amount of cobalt in the composition is increased.Thus for example, when the cobalt concentration is less than 12% byweight, only a minute carbon deficiency, scarcely, scarcely measurableby analytical means, such as a carbonztungsten atomic ratio of 0.99,will provide sufficient tungsten to reach a concentration of an averageof 8% by weight in the cobalt phase. On the other hand, acarbon:tungsten ratio of 0.94 will provide up to an average of 24%tungsten in the cobalt. it is desirable that the free carbon content beas low as possible, preferably less than 0.15 percent.

CI. Eta phase With a deficiency of carbon, a part of the tungstencarbidecobalt bond may consist of eta phase, Co W C, although this isgenerally undesirable. Up to 10 or percent by weight of eta phase may bepresent in the binder phase isolated after removal of tungsten carbide,but less than 5% is preferred, since as much of the tungsten as possibleshould remain in solid solution in the cobalt and as little as possibleconsumed to form eta phase. The presence of tungsten dissolved in thecobalt binder phase is at least partly responsible for the unusualcombination of properties of the products of this invention.

e. Anisodimensional tungsten carbide One of the preferred products ofthis invention is a dense body comprising anisodimensional tungstencarbide platelets bonded with from 3 to by weight of heterogeneouscobalt-tungsten alloy.

The term isodimensional means having the same dimensions, whileanisodimensional means not having the same dimensions. A particle thatis isodimensional is therefore one having approximately equal length,breadth and width. The term isodiametric is employed in the same sense,an isodiametric particle being one having equal diameters when measuredin different directions. A sphere is perfectly isodiametric; a grain ofsand or of sugar is approximately isodiametric and can also be describedas being isodimensional. The size and shape of ultimate particles andtheir arrangement in aggregates is more fully described by Dr. A. VonBuzagh, in Colloid Systems, published by the Technical Press, Ltd.(London, 1937).

Finely divided tungsten carbide of the prior art has ordinarily beenobtained by pulverizing coarser crystals. The finely divided particlesso obtained are, broadly speaking, isodimensional. When milled tungstencarbide is bonded with metal by the processes of the prior art to formhard, cemented carbide bodies, there occurs a recrystallization andgrain growth of the tungsten carbide. By metallographic methods, thesize and shape of the resulting carbide grains can be observed. A reviewof published micrographs of the grain structure of commercial cementedcarbides, as well as examination of a range of cobalt-bonded tungstencarbide products of commerce, indicates that the tungsten carbide grainsare isodimensional. While in some instances the polished cross sectionsof individual grains indicate a length or maximum dimension two or eventhree times that of the minimum dimension, this is the exception ratherthan the rule. In micrographs, grains give the impression of beinganisodimensional when a substantial proportion of the grains show amaximum dimension at least three times that of the minimum dimension.

For purposes of this invention, anisodimensional particles are thereforethose having a maximum dimension at least three times that of theirminimum dimension. Some of the products of the present invention consistlargely of anisodimensional tungsten carbide crystals of which themaximum dimension is at least three and preferably at least four timesthat of the minimum dimension. In such products the tungsten monocarbidegrains, which appear to be crystals, are typically present as triangularplatelets, the thickness of which is no more than one-fourth and usuallyno more than one-sixth the length of the side of the platelet. Preferredanisodimensional tungsten carbide particles are from 0.05 to 1 micron inthickness and from 0.2 to 4 microns in length or breadth. The commonestparticles are triangular platelets, although polygonal platelets arealso observed.

To obtain a body containing tungsten carbide platelets, the physicalstate of the starting powder is important. The tungsten carbide in thepowder admixed with cobalt before being heated must have a crystal sizeof less than 0.1 microns, and preferably has a crystal size of less than0.05 microns as calculated from X-ray line broadening or specificsurface area.

f. Impurities Foreign materials such as organic dirt, mineral dust orfragments of enamel or glass such as from equipment should bescrupulously avoided in preparing the bodies of this invention. Organicmatter can result in holes or inclusions of carbon in the final body andmineral materials such as silicates leave inclusions of glass which isvery harmful because the inclusions cause localized internal stressesupon cooling, thereby contributing to brittleness. Other mineral dust aswell as glass or enamel fragments are similarly deleterious. Localizedcarburization of powder during manufacture results in regions high incarbon in the final structure. If a portion of powder, such as the outerlayer of powder inclosed in a graphite mold, is carburized before it ispressed, there may be found homogeneous regions near the surface ofpressed pieces that contain an excess of carbon and show excessive graingrowth. Such homogeneously carbon-rich regions due to grosscontamination with carbon are to be avoided and they do not correspondto the type of heterogeneity of regions found throughout the structuresof the present invention.

g. Properties of the compositions 1. Acid resistance As shown by Kubota,Isheda and Ram in the reference mentioned above, in cobalt-bondedtungsten carbide bodies of the prior art in which the distribution oftungsten in cobalt is apparently homogeneous, a small decrease in theatomic ratio of carbon2tungsten to a ratio less than 1.0 remarkablyincreases resistance of the metal phase to dissolution in hydrochloricacid. This is believed due to the increased amount of tungsten in solidsolution in the cobalt phase.

In the products of the present invention, while some regions of cobaltare rich in tungsten, others are low in tungsten and are therefore notacid resistant. The acid penetrates the structures via these non-acidresistant regions, with the result that the overall acid resistance islow, in spite of a relatively high average tungsten content in thecobalt.

The amount of tungsten in solid solution in the cobalt can be determinedas described by Kubota, et al. A preferred method for measuring theamount of tungsten in the cobalt is described hereinafter in the methodsof characterization. The dense bodies of the prior art disclosed incopending application Ser. No. 660,986, which contain an average of morethan 8% tungsten in the cobalt, are characterized as having a resistanceto etching, R, of greater than 50 hours, where resistance is expressedin terms of number of hours required at room temperature forconcentrated hydrochloric acid to remove 0.25 milligrams of metal persquare centimeter of surface area per percent of metal present in theoriginal sample.

The dense bodies of the present invention, which contain an average offrom to tungsten in the cobalt phase are low in acid resistance,characteristically having an acid resistance of less than 50 andgenerally less than hours. Bodies in which the acid resistance is over50 hours are termed acid resistant." Bodies of the present invention arenot acid-resistant."

2. Strength The unusual strength of the dense bodies of this inventionis described in greater detail in the subsequent sections. Much of thestrength of the dense bodies of this invention is of course attributableto the skeletal strength of the tungsten carbide, however, the cobaltphase also quite evidently contributes substantially to the overallstrength.

Characteristically, a body of this invention containing about 12 percentcobalt has a transverse rupture strength of about 500,000 psi andpossesses a carbide skeleton with a strength of over 60,000 psi. Mostcommercial tungsten carbide bodies of the same cobalt contentcharacteristically will have a transverse rupture strength of only380,000 psi and a skeleton with a strength of about 46,000 psi.

Removal of the tungsten carbide from the dense bodies of this inventionby anodic etching leaves a coherent but porous and weak metal structure.Conversely, removal of the metal from the dense bodies of this inventionleaves a porous body of tungsten carbide which has a much lowertransverse rupture strength than before removal of the metal. It is thecombination of the interpenetrating metal and carbide phase whichprovide the high strength.

3. Hardness The hardness of the dense bodies of this invention, measuredat ordinary and high temperatures is higher than that of many of theprior art tungsten carbide bodies of equivalent cobalt content. This isone of the most important characteristics of the bodies of thisinvention. High hardness at high temperatures is of special value incutting tools. A representative dense body of this invention containing10 to 12 percent cobalt will measure 87 on the Rockwell A scale at800C., while most commercially available tungsten carbide bodiesprepared by methods of the prior art containing 12% cobalt have ahardness of only about 75 Rockwell A, and many commercially availablecarbides containing as little as 6% cobalt have a hardness ofonly 83Rockwell A.

The unusual hardness of bodies of this invention is largely dependentupon the structure of the tungsten carbide skeleton which bears most ofthe load in the composite body. The hardness increases with finer grainsize of tungsten carbide in the carbide skeleton. In the compositions ofthis invention the hardness is not appreciably reduced by the fact thatthe tungsten content in the cobalt phase is not uniform, so long as overhalf of the metal binder contains more than 8% tungsten. This is truebecause hardness is mainly determined by the grain size and coherentnature of the tungsten carbide phase.

4. Density The relation between the apparent density of bodies of thisinvention and their theoretical density as calculated from the volumesand individual densities of the components, permits an estimate of theinternal porosity. The bodies of this invention have an apparent densityof over 98% of the theoretical density, and preferably at least 99% ofthe theoretical density. Expressed in another way, the volume of a givenweight of a preferred body of this invention is generally equal to thesum of the volumes of the components calculated from the weight of eachcomponent divided by its density.

2. PREPARATION OF COMPOSITIONS OF THIS INVENTION a. Preparation of thePowder Mixtures 1. Starting materials Starting materials for use in thisinvention are tungsten carbide and cobalt which are substantially pure,that is, containing no more extraneous matter than is found in thetungsten carbide and cobalt powders conventionally employed in makingcobalt-bonded tungsten carbide cutting tools. Small amounts of iron, upto 0.5%, may be present from erosion of process equipment; but otherthan iron, the total impurities amount to less than 0.5% by weight, andpreferably are present only in spectroscopically detectable amounts.

Suitable colloidally subdivided tungsten carbide powder is described incopending application Ser. No. 772,810 filed Nov. 1, 1968, nowabandoned. This tungsten carbide is in the form of crystallites ofcolloidal size well under half a micron in diameter, typically 30 or 40millimicrons in diameter, the crystallites being linked together inporous aggregates, prepared by forming and precipitating tungstencarbide from a reaction medium of molten salt.

Cobalt suitable for use in this invention includes any source of cobaltmetal which can be used to prepare an interdispersion of cobalt withtungsten carbide powder; for example, finely divided powder such asCobalt F," sold by the Welded Carbide Tool Co. The metal is preferablymore than 99.5% pure cobalt, and should be free from impurities thatwould be harmful to the properties of cemented tungsten carbide.

2. Blending Components The cobalt and tungsten carbide powders suitableto be used in this invention must be intimately mixed. Extensive millingof the tungsten carbide with the metal is ordinarily employed to achievean intimate mixture.

It is preferred to use a mill and grinding material from which anegligible amount of metal is removed, and it is usually preferred touse ballmills or similar rotating or vibrating mills. Suitable materialsof construction for such mills are steel, stainless steel, or millslined with cobalt-bonded tungsten carbide. The grinding medium, which ismore susceptible to wear than the mill itself, should be of a hard,wear-resistant material such as a metal-bonded tungsten carbide.Cobalt-bonded tungsten carbide containing about 6% cobalt is a preferredgrinding medium. The grinding medium can be in various forms as balls orshort cylindrical rods about one-eighth to one-quarter inch in diameter,which have been previously conditioned by running in a mill in a liquidmedium for several weeks until the rate of wear is less than 0.01% lossin weight per day. Mill loadings and rotational speeds should beoptimized as will be apparent to those skilled in the art.

In order to avoid caking of the solids on the side of the mill, asufficient amount of an inert liquid medium is ordinarily used to give athin slurry of the tungsten carbide powder charged to the mill. One ofthe liquid media which are suitable for this purpose is acetone.

Ballmilling tungsten carbide in the presence of cobalt reduces theparticle size of the tungsten carbide and distributes the cobaltuniformly among the fine particles of carbide. It is often advantageousto have at least 25% of the carbide smaller than a micron, and mostpreferably the average particle size is less than a micron. When it isnecessary to reduce the particle size of the tungsten carbide it ispreferred to mill the tungsten carbide separately prior to interspersingthe carbide with cobalt. It is advantageous to start with the preferredcolloidal tungsten carbide disclosed in copending application Ser. No.772,810 referred to above, since it is not necessary to mill thetungsten carbide before it is milled with cobalt.

Milling of cobalt/tungsten carbide mixtures is continued until thecobalt is homogeneously interspersed with the finely divided tungstencarbide. Homogeneous interspersion is evidenced by the fact that it isessentially impossible to separate the cobalt from the tungsten carbideby physical means such as sedimentation or a magnetic field.

The mill is ordinarily fitted with suitable attachments to enable it tobe discharged by pressurizing it with an inert gas. The grindingmaterial can be retained in the mill by means of a suitable screen overthe exit port. The liquid medium is separated from the milled powdersuch as by distillation and the powder is then dried under vacuum.Alternatively the solvent can be distilled off directly from the mill.The dry powder is then crushed and screened, while maintaining anoxygenfree atmosphere such as with nitrogen or argon, or by maintaininga vacuum.

3. Adjusting the CarbonzTungsten Ratio Various means are known in theart for adjusting the ratio of carbon:tungsten in cobalt/tungstencarbide compositions. Thus, the ratio can be adjusted by simply addingsuitable amounts of finely divided tungsten, ditungsten carbide, orcarbon to the mill. For the purposes of this invention, it is necessaryto produce a carbon deficiency in the powder compositions which willresult in carbon deficient regions in the dense bodies. The term carbondeficient" will be understood to mean containing less than one atom ofcarbon per atom of tungsten after consolidation at l,300 to 1,500C.Similarly when referring to the atomic ratio of carbon:tungsten" of apowder, it will be understood that this means the atomic ratio afterconsolidation at high temperature. In other words, although the powderhas some carbon:tungsten ratio, it is not this ratio that is significantas it changes during heating. Thus, it is the ratio after heating whichis significant.

Carbon deficiency can be produced in tungsten carbide or mixtures oftungsten carbide and cobalt binders by a. synthesizing tungsten carbideof colloidal particle size such that the surface of the particlesconsists mainly of tungsten atoms which are not accompanied bycorresponding carbon atoms.

b. making a composition of tungsten monocarbide intermingled withditungsten carbide or finely divided tungsten metal or phases such as CoW C or eta phase, in which there is less than one carbon atom pertungsten atom.

. oxidizing part of the tungsten or intermingled cobalt to an oxidizedfrom which during subsequent heating with the remaining tungstenmonocarbide reacts to form carbon oxides which escape leaving carbondeficient regions in the final product corresponding to the oxidizedregions.

If only a small carbon deficiency, such as an atomic ratio ofcarbon:tungsten of 0.97 or 0.99 is to be created, small amounts of othermetals such as tantalum or titanium can be used in place of tungsten.However, in determining the carbonztungsten ratio in final compositions,the presence of such added metals or their carbides must be taken intoaccount. Of titanium and tantalum, it is preferred to use tantalumbecause its carbide acts as a grain growth inhibitor, and enhanceshardness at high temperature.

4. Heterogeneity in the powder Means for deliberately producingheterogeneity or local variations in the carbon:tungsten ratio have notbeen described in the prior art. Such variations can be produced by anyof the following methods:

Blending dissimilar powders A powder mixture of tungsten carbide andcobalt which is carbon deficient can be blended with a tungsten carbideor cobalt/tungsten carbide powder mixture which contains a theoreticalamount or a slight excess of carbon over that required to form tungstenmonocarbide, and then the blend is consolidated at high temperature. Forexample, the carbon deficient powder can be a mixture of tungstencarbide and cobalt which has been milled to develop a specific surfacearea in excess of three square meters/gram which is permitted to absorboxygen; this can be blended with a powder which is not carbon deficient,such as a milled powder of the tungsten carbide and cobalt of the priorart commonly used for producing cemented tungsten carbide bodies with anatomic ratio of carbon to tungsten of from 1.0 to 1.03, as commonlyemployed in carbide cutting tools. The carbon deficient powders can alsobe prepared by ballmilling a composition of cobalt and tungsten carbidealong with finely divided tungsten powder to provide the carbondeficiency; this powder can be blended as described with a powder whichis not carbon deficient. Powders having atomic C:W ratios as low as 0.80and as high as 1.1 can be used for blending.

Blending carbon with powder Another method for preparing a heterogeneouspowder comprises milling finely divided carbon with tungsten carbide andcobalt, preferably in sufficient amounts to produce a carbon tungstenatomic ratio of about 1.0. Generally more than 0.01 and less than 0.5percent by weight of carbon is added based on the weight of tungstencarbide. In the consolidated body the region around each carbon particleis carburized as the carbon dissolves, producing local regions which arenot carbon-deficient, the remainder of the body being carbondeficientand having a higher tungsten concentration in the cobalt. The tungstencarbide should have a C:W ratio of at least 0.8 prior to addition of thecarbon, and should have a ratio between 0.85 and 1.02 after addition ofthe carbon.

Many commercially available carbon blacks have a particle size in themillimicron range and any of these is a suitable source of carbon.Ordinarily it is preferred that the carbon be in a form which, after themilling step, will have a particle size of less than 5 microns and mostpreferably less than 1 micron.

Oxidizing the powder Still another method for producing heterogeneity inthe consolidated bodies comprises partially oxidizing a pelletizedpowder prepared by ballmilling cobalt and tungsten carbide containing aslight excess of carbon, subsequently'pelletized by tumbling, forexample, so that the outer surface of the pellets becomes more highlyoxidized than the interior. Thus a mixture of tungsten carbide andcobalt powders each from 1 to 10 microns in ultimate particle diametercan be ballmilled in an acetone medium for several days and then themixture can be removed from the mill and the powder dried withoutexposure to the air. A small amount of non-volatile organic matter fromthe acetone remains on the powder. The powder can then be screenedthrough a mechanically shaken screen of 60 meshes per inch undernitrogen, producing spherical pellets 10 to microns in diameter. Thepelleted powder will ordinarily be stored under nitrogen containing alow concentration of oxygen which is absorbed to an amount of from about0.1 up to about 1% by weight. The powders can alternatively be broughtslowly into the air providing the exposure is gradual enough to avoidlocal heating and excessive oxidation. Such an oxidized powder gives aconsolidated body containing an average atomic ratio of carbon totungsten of greater than 1.0, yet the body contains cobalt having morethan 8% by weight of tungsten in solid solution. If oxidation isexcessive, as much as 20% by weight of tungsten on the average is foundin the cobalt phase, and the acid resistance may approach 50 hours. Itis believed that the surface regions of the spherical pellets becomesmore highly oxidized than the interior, and that when the powder iscompressed and the composition is heated there results athree-dimensional continuum of carbon deficient composition derived fromthe surface regions of the pellets in which the cobalt binder phase isrich in dissolved tungsten, while those portions of the body derivedfrom the interior regions of the pellets remain as regions lessdeficient in carbon, containing little or no tungsten and having lowacid resistance. It is believed that these interiorderived regions ofthe composition reduce brittleness because of the ductility of the purecobalt binder.

Identification of the heterogeneous regions is sometimes difficult.However, by metallographic procedures, X-ray diffraction analysis,electrical resistivity measurements and Curie temperature measurements,regions high in carbon and cobalt regions low in tungsten can beidentified in the presence of regions low in carbon, and cobalt regionshigh in tungsten.

Heterogeneity preferably occurs only on a microscopic scale, but mayoccur in regions as large as a tenth of a millimeter. Thus, 50micron-sized granules of cobalt/tungsten carbide powder which have beenheated in hydrogen at 900 C. and have a carbon:tungsten ratio of 0.95can be blended with granules of a similar powder which have been heatedin hydrogen containing enough methane to deposit a small amount of freecarbon and have a carbonztungsten ratio of 1.03. Polished cross-sectionsof consolidated bodies made from such mixed powders show localizedregions high and low in carbon, about 50 microns in size, correspondingto the size of granules of the respective powders.

Preferred powders are those which produce bodies in which theheterogeneous regions are so fine and intermixed that they cannot beidentified under the microscope but are still known to be present fromeither X-ray diffraction patterns of the cobalt phase or from the factthat the acid resistance is lower than that of a similar body having thesame degree of porosity in which there is the same overall concentrationof tungsten in cobalt, but the tungsten is homogeneously distributed.Homogeneous distribution of tungsten in cobalt is attained when stepsare taken to eliminate the causes of heterogeneity as described above.

5. Reducing the powder When the dried milled mixture of tungsten carbideand cobalt contains over about 0.1 percent by weight of free carbon ormore than about 0.5 percent by weight of oxygen, it is preferred toremove these impurities by treatment at a minimum elevated temperaturein a very slightly carburizing atmosphere. Under these conditionsextreme local variations in carbon to tungsten ratio are corrected, butthe desirable variations within the limits of the present invention arenot affected.

Oxygen as well as excessive free carbon can be removed during thispurification, and at the same time the combined carbon content can beadjusted, all by heating the powder in a stream of hydrogen containing acarefully regulated concentration of methane. The powder can be chargedto shallow trays made from a high temperature alloy, such as lnconel,and the trays loaded directly from the inert atmosphere environment to atube furnace also made from Inconel or some similar high temperaturealloy.

The powder in a stream of the reducing gas is brought to a temperatureranging from 750 to l,OOC., depending on the metal content of thepowder, in from 3 to 5 hours, taking half an hour to raise thetemperature the last hundred degrees. For a cobalt content of about 1%,l,OO0C. is used, and for powders containing 12% cobalt, the temperatureis 800-900C.

The reducing gas should consist of a stream of hydrogen containingmethane and about percent of inert carrier gas such as argon. Thus, at1000C. the stream should contain 1 mole percent of methane in hydrogen;at 900 C., 2 mole percent of methane; and at 800C, 4 mole percent ofmethane in the hydrogen. The reduction/carburization at the maximumtemperature is carried on for a period of 0.5 to 3 hours, and aftercooling to room temperature under argon the powder is discharged to aninert atmosphere environment where it is screened through a seventy meshscreen. If desired this powder can be stored for extended periods insealed containers or it can be used directly in the next step of thisprocess.

Care must be employed to assure that in the reduction/carburization stepan excess of methane is avoided so that an undesirable amount of freecarbon is not introduced into the powder. It is to be noted thatalthough the reaction conditions are such that free tungsten metal wouldordinarily be converted to tungsten carbide, nevertheless very finelydivided tungsten carbide used in this invention remains slightlydeficient in carbon and is not carburized completely to a stoichiometricratio for tungsten carbide.

For compositions in which the desired atomic ratio of carbonztungsten isless than about 0.97, and where oxygen is to be removed by the foregoingreduction step, methane or other carburizing environments should beavoided and only hydrogen used. Generally speaking, with compositions ofhigher cobalt content, lower atomic ratios of carbonztungsten can beemployed. However, the minimum average atomic ratio of carbonztungsten,R is found to be R 1.0 0.0062(P-l where P is percent by weight ofcobalt.

An optimum ratio will be between this minimum and 1.02. Thus, for acomposition containing 10% by weight of cobalt, for example, the minimumratio is about 0.94. For a body containing 25% cobalt, the minimum ratiois about 0.85. A ratio above 0.90 is preferred. A maximum ratio, R formost purposes is R l-0.00166(Pl5). For a composition containing 3%cobalt the maximum ratio is about 1.02.

b. Consolidation of the powder The consolidated bodies of this inventionare prepared from interspersed cobalt/tungsten carbide powders.Generally speaking, consolidation is carried out in the manner describedin copending application Ser. No. 660,986, filed Aug. 16, 1967, Le. byheating and compressing the powders.

It is important that when the powder composition is being heated for thefirst time it should not be subjected to excessive pressure ormechanical constraint, especially when in a graphite or carboncontainer. Pressure can be applied providing it is not sufficient tokeep the sintering billet in intimate contact with the graphite walls ofthe mold. With some powders, a pressure of up to 1000 p.s.i. can beapplied during the heating step, since even under such pressure thebillet shrinks away from the mold and is not seriously carburized. Theharm that is caused by excessive compression may be due either toshearing forces which disturb the internal structure of the compositionat the beginning of recrystallization and sintering, or it may be due tochemical effects from contact with materiat such as graphite which isordinarily used to apply the pressure. Thus it has been observed thatapplication of pressure to the composition while in an alumina mold isless harmful to the resultant bodies, even using pressures higher than1000 p.s.i. The harm also may be due to trapping of gases in pores thatare collapsed by the pressure. In the absence of pressure such poreswould not normally become closed at this stage of sintering.

If the powder is first heated without application of pressure to aprescribed temperature it can thereafter be consolidated to density andmolded by hot pressing in a carbon mold without absorbing undesirableamounts of carbon. We have found that after the tungsten has dissolvedin the cobalt phase during the heat treatment it is much less readilycarburized.

Heat treatment is carried out in an inert atmosphere or in a vacuum. Aninert atmosphere is one that does not react with the powder, such asargon or hydrogen. Heat treatment is carried out at a temperature Twhich is above 1000C., but generally below the final consolidatingtemperature, T,,,, and the treatment lasts for about t, to 20 t,minutes, where:

log t 13250/( T, 273) 8.2 minutes and 6.5 l0g10 (P- 0.0039 ilOO C.

where P percent by weight of metal in the composition.

Thus the composition is heated to temperature T, and held for a minimumof t, minutes. The maximum time of heating is not critical attemperatures below which no appreciable grain growth of tungsten carbideoccurs, namely below about 1200C. However, above l,200C., the timeshould not exceed about 20 t,. For example, at l000C., it is necessaryto heat for at least 2% hours and preferably several times this long; at1 C. the composition is heated for at least 13 minutes; at 1200C. thehold time is a minimum of about 5 minutes and not over 2 hours; at1400C. the hold time is less than 10 minutes, and at 1500C. it is lessthan 4 minutes.

It should be noted that the temperatures and times required vary to someextent with the size of samples, dimensions of equipment, heating ratesattainable and the like. For example, it is possible to carry out theheating step either on loose powder or preconsolidated billet while thesample is being heated to the temperature at which it is to be finallyconsolidated. Such heating should be carried out rapidly in the rangeabove l,200C., providing the sample is heated relatively uniformlythroughout its volume. An integrated combination of temperatures andtimes equivalent to the fixed times and temperatures described, is inkeeping with the spirit of the invention, and will be apparent to thoseskilled in the art.

A preferred method of fabrication is by hot pressing the powders in themanner described below. Various types of hot pressing equipment areknown in the art. Depending on press design and desired operatingcharacteristics, heating can be by resistance heating, inductionheating, or plasma torch heating. Short heating times of a few secondsduration are attainable by resistance sintering under pressure.

Temperature can be measured very near the sample itself by means of aradiation pyrometer and can be cross-checked for accuracy with anoptical pyrometer. Such instruments should be calibrated against primarystandards and against thermocouples positioned in the sample itself sothat actual sample temperatures can be determined from their readings.Automatic control of heat-up rate and desired temperature can beachieved by appropriate coupling mechanisms between a radiant pyrometerand the power source.

The mold can be of a variety of shapes but is usually cylindrical, witha wall thickness of up to an inch or more. It is particularlyadvantageous to use a cylinder with a cross-section which is circular onthe outside and square in the inside in pressing bodies to be used ascutting-tip inserts, thereby fabricating them as near as possible totheir final desired dimensions.

As an example, for a 1 inch in diameter finished pressed round disc, theshell is cylindrical, 1 inch in inside diameter, 1% inches in outsidediameter, 4 inches in length. Thin graphite discs one-fourth inch inthickness and 1 inch in diameter are loaded in the cylinder on top andbottom of the material to be pressed. The surface of the graphite discsin contact with the sample can have a conical depression one-eighth inchin diameter at the center to form a tip on the sample and keep itpositioned in the center of the mold when it shrinks away from the sidesdue to sintering. Graphite pistons 1 inch in diameter and 2 inches longare loaded in both ends of the cylinder in contact with the one-fourthinch discs and protruding from the cylinder.

Graphite parts used in the press tend to oxidize at the pressingtemperatures used, and it is therefore necessary to maintain anon-oxidizing atmosphere or vacuum within the press. In addition toprolonging the life of the graphite parts, the use of a vacuum or aninert atmosphere makes it possible to remove the mold containing the hotpressed body from the heart of the induction heated furnace and cool thesample much more quickly than if it were left to cool in the hot zone ofthe furnace after shutting off the power. The press can be arranged topermit the mold to be removed from the hot furnace, and when this isdone the mold cools very rapidly by radiation. Thus the mold describedabove, removed from the furnace at 1400C., cools to dull red heat, about800C, in about 3 minutes.

Powders which are pyrophoric or absorb oxygen upon exposure to air,should be loaded into the mold in a non-oxidizing atmosphere, forexample in a glove box filled with inert gas. The appropriate discs andpistons can then be inserted and the loaded mold can be handled with thecontained powder essentially loosely packed or, for example, with nomore pressure than can be applied to the pistons with the fingers.However, it is often convenient to apply about 200 to 400 psi. pressurewith a small press, to give a more compacted sample for greatest ease inhandling.

in a preferred aspect of this invention, a cobalt/colloidal tungstencarbide powder mixture is pressed at about 200 psi as it is loaded intothe mold, it is then brought to the maximum temperature with no pressureon the pistons, and held for 2 to 5 minutes at maximum temperaturebefore applying any pressure. During the period at maximum temperaturewith no pressure applied, the body shrinks due to sintering. At the endof the period, the body attains -90% of theoretical density and itsdiameter is about 60% of the mold diameter. The pressure is thenapplied, reaching maximum in l5 to 30 seconds, and the presintered bodyis reformed into conformity with the mold. Maximum pressure andtemperature are applied until complete densification is attained, asindicated when movement of the rams ceases. This ordinarily does notrequire more than 5 minutes, and usually only one minute, after whichthe sample is immediately removed from the hot zone and permitted tocool rapidly by radiation to below 800C. in about 5 minutes or less.

The conditions which give rise to the preferred dense cobalt-bondedbodies are quite important and should be precisely established for theparticular composition and the type of structure desired.

Unduly long presintering times before application of pressure can beharmful due to excessive crystallite growth and the development of tooextensive and rigid a cross-linked carbide structure. Too early anapplication of pressure can also be harmful as pointed out above.Holding the sample for too long a time at maximum temperature shouldalso be avoided, not only because of a tendency towards carburizationbut also because secondary crystallite growth tends to cause acoarsening of the structure and eventually the development of porosity.Cooling too slowly can also be detrimental if the sample remains at hightemperature long enough for undesirable crystallite growth andstructural changes to occur. These structural changes can includechanges in the composition of the cobalt binder phase. Thus with a lowcarbon content and the corresponding large amount of tungsten initiallyin the cobalt phase, precipitation of eta phase occurs at elevatedtemperatures. This can be minimized by brevity of hot pressing andrapidity of cooling of the pressed product. Generally speaking, it isundesirable to have more than about 20% by weight of eta phase in thebinder, and it is preferred to have less than 5% eta phase in thebinder.

While it is preferred that the products of this invention be made byheating and sintering lightly compacted finely divided cobalt/tungstencarbide powders, followed immediately by application of pressure, it issometimes desirable to carry out the sintering step as a separateoperation.

Thus, in order to achieve maximum productivity from a hot press, theinitial sintering step can be carried out in a separate furnace in aninert atmosphere. This can be accomplished in several ways. For example,the starting powder can be loaded or lightly compacted into molds to belater used for hot pressing, and then heated rapidly in an inertatmosphere to a temperature within from 50 to 200 of the final hotpressing temperature to be employed. The mold and its partially sinteredcontents, while still hot, can be passed directly into a hot pressingoperation.

The maximum temperature at which the bodies should be pressed is largelydependent on the cobalt content, although the proper temperature is tosome extent dependent on the size of the molded piece, the heating rate,and the available pressure as well. The compositions of this inventionare conveniently subjected to a temperature of T,, for a period of r to20 t, minutes, where heated graphite block, the limiting factor beingthe rate of heat transfer from the graphite equipment via the mold tothe sample. Rapidity of heating is especially important in compositionswhich have an atomic ratio of carbon:tungsten close to 1.0.

Pressure can be applied to the cobalt/tungsten carbide composition in ahot press through the action of remotely controlled hydraulic pneumaticrams. Applying pressure simultaneously through two rams to the top andbottom gives more uniform pressure distribution within the sample thandoes applying pressure through only one ram. An indicator can beattached to each ram to show the amount of ram movement, therebyallowing control of sample position within the heat field and indicatingthe amount of sample compaction. The end section of the rams, which areexposed to the high temperature zone should be made from graphite.

A variation of 100 from the mean specified temperature provides to someextent for the variables mentioned above. Thus, in order to attaintemperature equilibrium in the interior without overheating theexterior, larger bodies require a lower temperature, which also permitsa longer heating time. Higher temperatures and shorter times can beemployed when high molding pressures can be used and smaller moldedbodies are being made.

The most important factor in determining consolidation conditions is thephysical nature of the heat-treated composition of the invention. Whenthe composition is a heat-treated powder, for example, it can be loadedinto graphite molds and heat and pressure simultaneously applied untilthe material reaches the recommended temperature range, T,, at which thepressure is maintained for the specified time. The required pressure maybe as low as 100 to 200 pounds per square inch for compositions such asthose containing 15 to 25 percent by weight of cobalt and which are softat the pressing temperature. Several thousands of pounds per square inchis required for bodies containing one to three percent cobalt, althoughpressures of not more than 4000 pounds per square inch are usually usedwhere operations are in graphite equipment.

For compositions containing from to 12 percent cobalt the requiredpressure can also vary according to the physical nature of thecomposition. Thus if a sintered powder composition is used, which hasbeen heat-treated at a temperature T, close to the maximum allowabletemperature T,,,, a high pressure such as 4000 p.s.i. is preferablyapplied over a prolonged period, often continuously, while the mass isheated from l000C. to temperature T,,,.

On the other hand, if degassed powder is preconsolidated to relativelyhigh density such as about 50 percent of theoretical density, so thatvoids or pores larger than about ten microns are eliminated, and thiscompact is then heat-treated at temperature T it shrinks spontaneouslyto a coherent body. If T, is then raised to T sintering continues and arelatively dense body is obtained which can then be molded by briefapplication of pressure at temperature T,,,.

Compositions of the invention require application of pressure at thedefined maximum temperature, T,,,, to eliminate voids. In such instancesthe consolidation is carried out until the body of the invention reachesa density of greater than 9 percent and preferably greater than 99percent of theoretical, corresponding to a porosity of less than onepercent by volume. However, for many uses even this degree of porositymay be too high. The porosity of the bodies of this invention ischaracterized by preparing polished cross-sections of the bodies forexamination under a metallurgical microscope. Pores observed in this wayare classified according to a standard method recommended by theAmerican Society for Testing Materials (ASTM) and described on pp. 1 16to 120 in the book entitled Cemented Carbides, published by the Mac-Millan Company of New York 1960). Thus, bodies of this invention arepreferably pressed until a porosity rating of A-l is obtained especiallywhere the material is to be subjected to heavy impact or compression.This corresponds to a density of essentially 100% of theoretical or avolume porosity of about 0.1%. However, porosities as great as A-3 orA-4 are suitable for many uses, since such bodies nevertheless have veryhigh transverse bending strength. Even a porosity rating of A-5 whichcorresponds to a density of about 98 percent and a porosity around 2percent, is acceptable for the compositions of this invention.

Pressures of from 500 to 6000 psi can be used in graphite equipment, butgenerally speaking not over 4000 psi can be applied without danger ofbreaking the equipment, unless the graphite mold and plungers arereinforced with a refractory metal such as tungsten or molybdenum.

Instead of loading a powder into a mold, preconsolidated compacts in theform of billets can be prepared and heattreated and then loaded in amold for hot pressing. Such heattreated, sintered billets can also beshaped by rolling or forging in an inert atmosphere.

After final consolidation to a dense billet the compositions ofthisinvention can be further shaped by bending, swaging, or forging at abouttemperature T,,,. Similarly, pieces can be welded together by bringingtwo clean surfaces together under pressure.

3. CHARACTERIZATION OF DENSE COMPOSlTlONS a. Chemical analysis Thechemical composition of the bodies of this invention can be determinedby conventional chemical analysis for the elementary constituents.Samples can be pulverized as in a Plattner steel mortar and screenedbefore sampling for analysis. The more convenient methods of analysisfor tungsten, cobalt, total carbon, free carbon, oxygen, and density aredescribed in copending application Ser. No. 660,986 referred to above.

b. Examination with optical microscope To examine homogeneity of theoverall structure and detect gross inclusions or localized coarse grainstructure polished surfaces can be examined quite satisfactorily atmagnification up to 2000X with the light microscope. In order to examineindividual tungsten carbide grains and their structural arrangement inconsolidated bodies, it is advantageous to fracture a sample and examinethe fractured surface or to etch the polished surface with chemicalagents which due to the different rates of chemical attack dissolve athin layer from the exposed grains, enhancing the contrast between thetungsten carbide and metal phases and making grain boundaries morereadily visible. Techniques commonly used for preparing fractured andetched surfaces and analysis of those surfaces are fully described incopending application Ser. No. 660,986 referred to above.

c. Examination with electron microscope Because of the unusuallyfine-grained structure, especially in preferred bodies of the inventionin which over half of the grains of tungsten carbide are less than 0.75microns in diameter, it is necessary to use the electron microscope tomeasure the grain size. To measure the grain size of tungsten carbideboth the boundaries between tungsten carbide grains and the tungstencarbide-metal phase boundaries must be outlined. Furthermore, the metalphase must be distinguished from tungsten carbide so that the former canbe avoided when counting the grain size of tungsten carbide. Amulti-step chemical etch accomplishes this objective. The proceduredescribed in copending application Ser. No. 660,986 referred to above isemployed in characterizing the products of this invention.

d. Transverse rupture strength Many suitable procedures have beendescribed in the literature for the measurement of transverse rupturestrength. We prefer to use the method described in application Ser. No.660,986 referred to above.

e. Magnetic characteristics The Aminco-Brenner Magne-Gage," basically atorsion balance made by the American Instrument Company, Silver Springs,Maryland, is a device which permits quantitative determination of therelative force required to pull a magnet away from a specimen containingmagnetic material.

Use of the Magne-Gage and preparation of samples for analysis are fullydescribed in application Ser. No. 660,986 referred to above.

f. Acid resistance The method of measuring the acid resistance ofmetalbonded tungsten carbide bodies is also described in applicationSer. No. 660,986 referred to above As pointed out there, the samples tobe tested are cut into small bars 0.006 X 0.006 X 0.55 inches. Thesample bars are then cleaned and measured to the nearest 0.001 inch,weighed to the nearest tenth of a milligram and suspended individuallyfrom a glass rod so that the bars hand about 1 inch below the rod.

The surfaces of the bars are then cleaned again by suspending the barsin boiling trichloroethylene and then washing them with water andacetone. The bars and their support wires are then weighed to thenearest tenth of a milligram and the bars are then immersed in 25C.hydrochloric acid containing 35% by weight of hydrogen chloride. 50milliliters of acid are used for each bar, and the acid is agitatedthroughout the test. The samples are removed periodically and aremeasured and weighed.

Acid etch resistance R is expressed in terms of the number of hoursrequired for the acid to remove 0.25 milligrams per square centimeter ofsurface area per percent of metal originally present in the sample.

In measuring acid resistance it is important that the surfaces of thetest samples be clean and smooth, free from roughness or scratches. Itis also important that the samples be free of cracks and porous defectswhich lead to low values for R. The pores provide avenues of attack ofthe cobalt phase by the acid, so that by the above method the acidresistance may appear to be irregularly low. In such instances, thepores can be filled with a resin or wax by impregnating the specimens,for example in hot beeswax, the excess wax is then wiped from thesurface with a cloth soaked in acetone and the outer surface of thespecimen further cleaned with an aqueous detergent in an ultrasoniccleaning device until the surface is water-wettable, indicating that waxhas been removed from the exterior. By this procedure, the fine poresremain blocked and a true value of acid resistance can then be obtained.

g. Tungsten content of the cobalt A preferred method for measuring thetungsten content of the cobalt is to 1) polish a section of sample; 2)remove tungsten carbide by anodic etching for an hour in a solutioncontaining percent by weight of potassium hydroxide and ten percent ofpotassium ferricyanide; 3) rinse; 4) remove the residual metal binderlayer by dissolving it in a ten percent solution of hydrochloric acid;and 5) then again etch to remove tungsten carbide, thus leaving a filmof metal binder a few thousandths of an inch in thickness. The sample isthen examined by X-ray diffraction and the lattice constant of thecobalt determined. The percentage of tungsten in the cobalt iscalculated, based on the information given in Handbook of LatticeSpacings and Structure of Metals," Vol. 1, page 528, Pergamon Press,1958, by W. B. Pearson. When no tungsten is present, the latticeconstant of cubic cobalt is 3.545 angstroms, and when the initial bindercontains 21% by weight of tungsten and 79% by weight of cobalt in solidsolution, the lattice constant is 3.570.

We have found that the metal binder phase can be isolated byelectrolytically etching a body of the invention, using it as an anode,in the potassium hydroxide-potassium ferricyanide solution for 24 hoursat a current density of 3 amperes per square inch. then rinsing in waterand removing the layer of cobalt alloy, which is from 0.005 to 0.010inches thick, and drying at 60C. under nitrogen. The tungsten contentdeter mined by X-ray diffraction from powder patterns, correspondswithin the limit of error to the ratio of weights of tungsten totungsten plus cobalt, determined by chemical analysis, providing nosubstantial quantity of Co W or carbide phases are present. In thisrecovered metal phase, tungsten carbide and cobalt-tungsten carbidephases such as eta, Co W C are determined by heating the sample in 35%hydrochloric acid at C. for 1 hour, then filtering and weighing thewashed and dried insoluble residue which will contain the carbides whichare insoluble. If the intermetallic compound Co;,W is present, it willdissolve in the acid, but it is seldom present in the unannealed bodiesof this invention.

When eta phase is present in a body of this invention which is rapidlycooled, it is in a form rich in tungsten and corresponds to theconventional formula co,w,c which is re ported to have a face-centeredcubic lattice constant of 1 1.08 angstroms. However, when bodies of thepresent invention are slowly cooled from 1400 or 1300C. at 5C. perminute, the eta phase apparently absorbs cobalt or loses tungsten, sothat the ratio of cobalt to tungsten changes from 3:3 to 3:2, and thelattice constant changes continuously from 11.09 to 10.75 angstroms. Thelattice spacing of the eta phase, when present, serves to indicatewhether a body has been rapidly or slowly cooled.

h. Density The method of measuring apparent density should be selectedaccording to the type of specimen available. Most conveniently theactual density of any given composition is measured on a convenient sizesample by weighing the sample first in air and then immersed in waterpreviously boiled to remove dissolved air. The density is thencalculated from the equation:

d actual density in grams/cubic centimeter;

W weight in grams in the air;

W =weight in grams in the water; and

S specific gravity of water at the temperature of measurement.

The theoretical density of a composition is determined by the equation:

t= l563slcs+ 15.63 (-c) where t= theoretical density in grams/cubiccentimeter;

c weight percentage of tungsten carbide; and

s specific gravity of the tungsten-cobalt alloy binder phase.

Percentage of theoretical density is then calculated by the expressionpercentage of theoretical density d/t X 100.

A method for measuring actual density of'irregularly shaped specimensemploys mercury displacement, as described by Maczymillian Burke,Roczniki chem., 31, 293-295 (1957), Pykometer for Determining the BulkDensity of Porous Materials," and further referred to in J. Am. Chem.Soc., 45, (7), p. 352-353 (1962), by the same author.

i. Heterogeneity of tungsten in the cobalt Variations in theconcentration of tungsten in solid solution in the cobalt phase can beobserved by careful examination of the X-ray diffraction lines of thecubic cobalt phase of the recovered metal binder. When tungsten isuniformly dis-' tributed as in products of the prior art, the latticespacing of the cobalt is uniform as evidenced by sharp single peaks inthe diffraction lines of the cobalt, whereas in products of the presentinvention different regions of cobalt contain difierent amounts oftungsten in solid solution so that the diffraction lines, recorded asintensity versus diffraction angle show broadening, or shoulders due totwo or more unresolved peaks, or even two or more separate peaks,depending on the degree of resolution obtained and the irregularity ofdistribution of tungsten in the cobalt phase.

Attainment of resolution in X-ray diffraction is discussed in somedetail by Emmett F. Kaelble Handbook of X-ray, Mc-

, Graw Hill Book Co., (1967), pages 9-14 to 9-30.

Measurement of exact line position includes the technique ofincorporating into the sample of cobalt-tungsten binder a uniform amountof sodium chloride to serve as an internal standard. (Refer to H. P.Klug and L. E. Alexander, X-ray Diffraction Procedures, John Wiley &Sons, Inc., N.Y. (3rd printing 1962) pages 452-3.) By this means themeasured angle two-theta for a cobalt line from cubic cobalt iscorrected by the difference between the measured angle for a neighboringsodium chloride line and its known standard value. From the correctedvalue of two-theta for the cobalt line, the unit cell dimension of thecobalt is calculated as on p. 343 of the foregoing reference.

The tungsten carbide-cobalt composition of this invention is cut orground to produce a specimen with a smooth surface having an area ofseveral square centimeters. The exposed surface must be representativeof the interior of the specimen, outer layers which might be oxidized orcarburized by previous treatments being ground away to a depth of atleast 0.06 inches.

The method consists in anodically etching the smooth, cleaned surfacefor 24 hours in alkaline ferricyanide solution to remove the tungstencarbide phase to a depth of up to 1/32 inch, scraping off and recoveringthe residual, porous cobalt phase, and examining it by X-raydiffraction.

Variations in the d spacing for the strongest cobalt line indicatevariations in the amount of tungsten in solid solution in the cobaltlattice; and the ratio of the intensity of the strongest eta line tothat of the strongest cobalt line, serves as an empirical indication ofthe relative amount of eta phase present in the cobalt. This is calledthe eta ratio.

It appears that the eta phase is precipitated within the cobalt metalphase, since it is not removed by the anodic attack which removestungsten carbide, in spite of the fact that, by itself, eta phase isquite soluble in this reagent. Similarly, tungsten within the cobaltlattice is not attacked by the anodic etch.

While not ordinarily encountered, destruction of eta phase may occurduring anodic attack if most of the cobalt has been converted to etaphase, so that little of the cobalt is left to surround and protect theeta phase. Also, is some unusual specimens of cobalt-bonded tungstencarbide, other than that of Ser. No. 660,986 referred to above, whenmuch eta phase is present the cobalt may be very finely divided, so thatthe recovered powder is oxidized in air and partially destroyed. Thesefactors have been taken into account in devising the procedure describedbelow.

The surface is cleaned by immersing in boiling dimethylformamidefollowed by rinsing in acetone and drying. Alternatively, the sample maybe held over a gas flame until it is just red hot and permitted to coolslowly, after which it is scrubbed with steel wool in water and dried.

Electrical contact is made with the sample by wrapping a fine platinumwire around the specimen or using platinum clips. The lead-in wire forelectrical contact should be covered with rubber insulation. The sampleis hung in a 100 cubic centimeter plastic or glass beaker and connectedto the positive source of direct current, thus making the specimen ananode.

A cathode of platinum sheet 1 inch X is inch welded to a platinum leadwire is also hung inside the wall of the beaker using the wire as a hookand connected to the negative source of direct current.

Up to four cell-assemblies of this type can be connected in series to a12 volt DC source. The specimens are connected toward the positiveterminal of a 12 volt storage battery, or other source providing acurrent of up to l ampere.

The electrolyte is made by dissolving 100 grams of potassiumferricyanide and 100 grams of potassium hydroxide, adding these to about30 ml. of distilled water and stirring until the mixture becomes hot,and then diluting to about 1 liter. Sufficient solution should be addedto each beaker to cover the sample and most of the cathode. Usuallyabout 75 milliliters of solution is required. The beakers may be coveredwith a sheet of plastic if desired, to reduce electrolyte spray duringelectrolysis which is continued for 24 hours with a current to eachspecimen of 0.7 amperes.

At the end of the electrolysis period, the specimen is removed andrinsed in water to remove alkali without losing any cobalt.

The cobalt within 1/32 inch of the edges of the cut surface, which havebeen in contact with graphite, is then trimmed away and discarded. Somespecimens yield cobalt films which are cracked and only very lightlyadherent. In such cases, the cobalt film can be removed from the centerof the cut surface leaving the cobalt around the edges.

The cobalt is collected by scraping it off under water in a pan, excesswater is decanted and the cobalt is washed with distilled water into a 3inch diameter porcelain mortar, along with about 10 milliliters ofwater. Excess water should be decanted from the mortar. The cobalt isthen ground by about 10 strokes of the porcelain pestle, to break up theflakes. Excessive grinding must be avoided, since it may affect thecobalt structure. The powder is then washed with distilled water into athin plastic bag held so that the cobalt will collect in one comer. Thecobalt in suspension is drawn toward the corner by holding the latternext to a small magnet. The cobalt is then held in the corner with themagnet while the water is decanted off and replaced with 10 millilitersof n-propyl alcohol. The powder is then suspended and again drawn to thecorner and the alcohol discarded.

The corner containing the cobalt is closed off by twisting it severaltimes, tying it loosely, and cutting off the rest of the bag.

Depending on the amount of cobalt in the original composition and thesize of the specimen, one or more preparations of this type from thesame piece of material may be required to obtain from 50 to 250milligrams of cobalt powder for examination. In repeating thepreparation the etched surface is well scraped, or preferablysand-blasted before being again anodically etched.

Two somewhat different procedures have also been employed in diffractionanalysis of the cobalt phase. They are referred to hereinafter asprocedure A and procedure B.

In procedure A, described more fully below, a 75 milligram sample isused with a parafocusing device to obtain a diffractometer tracing fromwhich the line position, shape, and intensity is employed to estimatethe percent tungsten in the cobalt, and the variation in the amount oftungsten in different portions of the sample, i.e., the heterogeneity ofdistribution of tungsten.

In procedure B, also described in detail below, a sample of greaterweight is used with a flat sample holder, automatic stop scanning every0.04 and accumulating the same number of counts at each point. The timeat each point is recorded on punch tape fed to a computer from which aprofile of intensity versus angle is constructed. The punch tape isconverted to punch cards fed to a computer programmed to calculateintensity profile at each point, then interpolate to a finer mesh ofpoints spaced apart by one-tenth the separation of alpha-l and alpha-2at that point, using a Lagrangian polynomial fit covering a range ofeight consecutive points, then applying the Keating correction for thealpha doublet whereby the alpha-2 contribution is subtracted from thetotal intensity at each point using a series approximation, thus leavingthe equivalent alpha-1 profile. From this the amount and distribution oftungsten in the cobalt is more accurately determined than by procedureA. Details of these procedures are as follows:

PROCEDURE A Apparatus North American Philips Diffractometer Power SupplyType No. 12045 X-ray Target Source Cobalt with iron filter to givecobalt alpha radiation Wide Range Goniometer/type No. 42202 ElectronicCircuit Panel Type No. 12049 Advance Metal Research Corp. AutofocusingAttachment,

Model No. 25-201 SAMPLE PREPARATION Thirty-five one hundreths of a gramof sodium chloride is ground in an agate mortar along with a 0.075 gramsample of the powder to be tested, and the mixture is screened through a325 mesh screen. The salt is present to provide representative peaks ofknown spacings at 1.99 and 1.628 angstroms. The screened powder isplaced on a fiberglass sample holder along with 0.2 milliliters of amylacetate and 1 drop of 25% collodian solution. These are mixed todisperse the powder and spread it to a film on the holder over an area 3inches long and 1 inch wide at the center and one-half inch at the ends.

INSTRUMENT CONDITION The X-ray apparatus is fitted with a cobalt targetand iron filter at 25 kilovolts and 20 milliamps. Scanning speeds of 5%"and W per minute are used. The chart speed is 30 minutes per hour. Thediversion slit is -4; the receiving slit 0.010-inch wide. Scintillationcounter detector, 950 volts, 6 volt base line, gain zero, scale factor8, multiplier 0.8 and time constant 4.

CALCULATION PROCEDURE The scans are examined, and the d spacingscorrected if necessary from the spacings of the known sodium chloridelines, corresponding to 53.3 26 corresponding to 1.994 d A.

Cobalt Lattice and Cobalt-Eta Co W C Ratio The region 48 to 52 20 isscanned at h" per minute. The counts per second for cobalt in the regioncorresponding to a d value of around 2.06 A. are recorded.

Cobalt Peak or Multiple Cobalt Peaks The region 51 20 to 54 20 isscanned at Vs" per minute. The location of the cobalt peak or multiplecobalt peaks and the sodium chloride internal standard are read indegrees 20. The alpha-1 and alpha-2 peaks are averaged for sodiumchloride and cobalt. A correction for the sodium chloride 20 degreelocation is appropriately made to the cobalt 20 degree location. Thecorrected (1 spacing is then calculated for the lattice constant orconstants of cobalt.

The amount of tungsten alloyed with the cobalt phase is calculated fromthe following linear relationship.

Products of this invention show the cobalt line with the 2.06

spread out or with pronounced shoulders or even with multiple peaks.This indicates that there are multiple cobalt-tungsten alloys presentwith different concentrations of tungsten in solid solution. It has beenobserved that when at least some of the cobalt contains less than 8%tungsten the product has a low resistance to removal of cobalt withhydrochloric acid even though the average concentration of tungsten iswell above 8% and even through the amount of alloy which contains lessthan 8% tungsten is a minor one.

If the tungsten is evenly distributed in the cobalt, all cobalt crystalshave the same lattice constant and the upper portion of the linecorresponding to a d-spacing of 2.06 angstroms is symmetrical about itspeak or maximum height, as shown in Curve A of FIG. 1, and its breadthat half height is about the same as that of the nearby sodium chlorideline as shown in Curves B and C of FIG. 1 which are symmetrical about acenter line.

On the other hand when the 2.06 angstrom line of cobalt clearly consistsof several peaks as in curves D and E of FIG. 1 it is evident that thereare different regions in the cobalt which contain two or more differentlevels of tungsten and thus have different lattice constants. In suchinstances, heterogeneity is obvious.

The small dimensions and even distribution of these heterogeneousregions is shown by cutting successive thin slices of the compositionand showing that in each the cobalt shows the same heterogeneity inregard to tungsten content.

When the cobalt peak shows a shoulder as in portion d of the curve F ofFIG. 1, as seen by comparison with the right side a of curve F, thecomponent peaks can be identified by inspection only after considerableexperience, but may be more easily located by a simplegraphical'procedure: a dotted line 0 is drawn to be symmetrical aboutthe midline between the 11-1 and 02-2 peaks at 5 1 .47 20 correspondingto a lattice constant of 3.568 angstroms. The X-ray radiation is notquite monochromatic and consists of two slightly different wavelengthswhich causes double peaks known as alpha-l (a-l) and alpha-2 (oz-2) foreach d-spacing. The symmetrical curve a,a would be characteristic ofcobalt containing a single homogenous concentration of 19 percenttu'ngsten. However, the curve d which is above 11,, indicates that someportions of the cobalt also contain lower concentrations of tungsten.The difierence in intensity at each angle, between curve d and a isplotted as a difference, curve bb,. A curve b, is now drawn symmetricalto the side b, about the line at angle 51.7". Then a new curve c isdrawn plotted as a difierence between the lower portion of curve d andcurve b This new curve 0 is centered at 5 19.

Thus, the curve da, can be empirically resolved into three componentpeaks centered at 51.47, 51.7 and 51.9 degrees. These peaks correspondto regions of cobalt containing 19%, 8% and 0% tungsten in the cobaltlattice, and the relative height of the peaks indicates the relativeamounts of the different regions present.

This method was employed on three different samples of cobalt recoveredfrom the same specimen of composition of this invention and thefollowing observations were reported indicating the reproducibility ofthe method:

Relative Curve G of FIG. 1 is the X-ray diffraction intensity curve ofthe strong line of a different cobalt sample obtained from the samecomposition used for curve F, and graphically analyzed the same way,giving essentially the same results. Curve H of FIG. 1 is a similargraphical analysis of the cobalt peak of another product of thisinvention.

In curves I and K of FIG. 1 the position of component peaks can be seenby inspection. Where the range of heterogeneity is still greater as in Jand L, while it is still possible to observe directly that numerouspeaks are present, it becomes more difficult to identify them, and in Mand N it is scarcely possible to say what components may be present.

The cobalt in compositions similar to those of this invention exceptthat the distribution of tungsten is homogeneous, is ordinarily notsufficiently fine-grained to cause line broadening. Thus, the shape ofthe peak is similar to that of the sodium chloride line used forcomparison. As the binder phase in the products of this invention isfine-grained the cobalt line in products of this invention is broadened,and the broadening is due to the presence of several component peakswith different d-spacings corresponding to different tungsten levels inthe cobalt, i.e., heterogeneity.

We have devised means for analyzing these curves into component peaks,simultaneously taking into account the alpha-1 and alpha-2 components.

PROCEDURE B The cobalt powder isolated by removal of the tungstencarbide as described above is mixed with an equal volume of sodiumchloride, and prepared as an X-ray diffraction powder sample. The X-raydata is gathered by an automatic stepscanning diffractometer, usingchromium radiation, and the intensity is recorded on a punched papertape. The tape is then converted to punched cards which are processedusing a digital computer. The entire section of the X-ray pattern isfirst corrected for the K11 doublet broadening using the methoddescribed by Keating (Rev. Sci. Inst. 30, 752 (1959)). The correctedpattern then gives the instrumental broadening in the form of the (220)sodium chloride peak, as well as the observed (1 l l) cobalt alloy peak.The limits of the sodium chloride peak are chosen as the points wherethe intensity dropped to the level of the background intensity. Thelimits of the cobalt alloy peak are chosen so that when the sodiumchloride pattern is superimposed at the extreme ends of the cobalt peak,the maximum of the sodium chloride peak falls in the low intensityregion of the cobalt peak. This is illustrated in FIG. 2 wherein l and 3represent the superimposed sodium chloride peaks and 2 represents thecobalt peak.

The sample broadening curve is then computed for all data points betweenthose defined by the two positions of the sodium chloride maximumdescribed above.

The computer program first subtracts the background from both peaks. Itthen computes the sample broadening using a method similar to thatdescribed by Patterson (Proc. Phys. Soc. A63, 477 (1950)). The programuses three different schemes to minimize the residuals of thediffraction peak. In phase one, it selects the largest availableresidual, and reduces it by If the corresponding subtraction or additionof the instrumental peak reduces the sum of the squares of theresiduals, the adjustment is accepted as part of the solution; if not,the step is reversed. In this and all other phases, the condition isimposed that the sample broadening curve may not be negative at anypoint. The points on either side of the maximum residual are thenreduced by 10% if such reduction decreases the sum of squares ofresiduals. When no further improvement can be made by reducing thelargest residual or its two neighbors, the program enters phase two,where each of the residuals is first increased, then decreased, in turn.If an adjustment produces a reduction in the overall sum ofsquares, itis made a part of the solution. If an improvement of the fit of thesolution is achieved during this process, the program returns to phaseone and begins again, if not, it goes on to phase three. This involvesthe simultaneous increase and decrease of adjacent pairs of residualswithin the available range. If improvement is made in the solution, theprogram returns to phase one and restarts; if no further improvement canbe made, it prints out the results.

In summary, the (1 l l) cobalt alloy peak and the (220) sodium chloridepeak are scanned, using chromium radiation. The diffraction pattern isrecorded and processed by Keatings technique to remove the K04 portionof the diffraction pattern. The sodium chloride peak is then used as aninstrument broadening profile, and is employed to determine the natureof the sample broadening present in the cobalt alloy peak, usingPattersons method. The relaxation takes place in three stages: first,the maximum residual and its adjacent neighbors are reduced; when thatis no longer effective, each of the residuals is reduced in turn;finally, adjacent pairs of residuals are increased and decreasedsimultaneously. This process then yields the profile which correspondsto the X-ray broadening of the cobalt-tungsten alloy, free of theeffects of instrumental broadening.

To correct for instrumental errors, all calculated profiles are shiftedto the angular position which would place the sodium chloride referencepeak at its proper diffraction angle. The

resulting peak positions for the cobalt phase recovered from 14cobalt-bonded tungsten carbide compositions are shown in FIG. 3,together with a plot of percent by weight of tungsten vs. peak position,and a histogram showing the number of peaks whose estimated ranges fallat a given point. The error range shown is 1 0.0l25 20 for each peak andzero for the sodium chloride peak (not shown). These data are thenreplotted, adjusting the relative positions of the sample patternswithin a range of 1 0.015 26, the estimated error in the determinationof the sodium chloride peak position. The relative positions of peaksfor a given sample are not changed, but are shifted as a unit. Thesedata are plotted in FIG. 4, with the error estimate increased to 10.0l75 20, together with the plot of tungsten concentration and thefrequency histogram. As may be seen, a strong correlation in the peakspacing is now evident from the histogram.

These data are itemized in Table I, which lists the peaks for eachsample, their indicated compositions, and the closest weight of tungstengrouping in the histogram. These histogram groupings are summarized inTable ll, stated in weight of tungsten, and converted to atomic oftungsten, together with possible atomic ratios. The occurrence ofintegral atomic ratios suggests the possibility of ordered structures atthe various compositions. Such order would seem to be required toexplain the segregation of composition in these alloys.

Of the 14 different compositions analyzed by the preceding procedure,and identified by the sample numbers 136C to 192C, it will be noted fromFIG. 4 that 6 out of the 14 did not contain tungsten-cobalt regionsgiving peak positions greater than about 67.8 26, and which thereforecontained no regions containing less than 8% by weight of tungsten inthe cobalt. These six compositions are therefore not examples ofcompositions of this invention. One of these six, numbered 158C,corresponds to the composition represented by Curve A of FIG. 1. Theremaining eight compositions are all heterogeneous, containing regionshaving less than 8% tungsten in the cobalt. Of these eight, number 184Acorresponds to the composition represented by Curve D of FIG. 1, whichis the composition of Example 4; 136C corresponds to Curve E of FIG. 1and to Example 192C 184C corresponds to Example 5; 192C corresponds toExample 7; 192B corresponds to Example 8; and 192A corresponds toExample 9.

TABLE I Closest Sample No. of Peaks wt% W Group F t- NN 14.0126 14.310.117 11.4 7.0:.7 7.5 184D 2 26.215 26.0 24.415 23.9 184E 2 25.415 26.024.11:.5 23.9 192A 6 1811.6 17.2 15.0;26 14.3 10.8127 11.4 7.3:.7 7.53.818 5.0 2.8:.8 1.5 1928 5 14.816 14.3 11.617 11.4 8.0:.7 7.5 5.0:.85.0 l.3:!:.9 1.5 192C 4 11.517 11.4 7.0:t.7 7.5 4.5:.8 5.0 1.4:.9 1.5

TABLE 11 Possible wt Group At W Atomic Ratio 26.013 10.12:.15 l/l(10.0%) 23.913 9.15:.13 1/11 (9.09%) 21.51.35 8.11:.16 1/12 (8.33%)17.214 6.24:.17 1/16 (6.25%) 14.314 5.08:.16 1/20 (5.00%) 11.4143.96:.15 l/25 (4.00%) 7.51.4 2.53:.14 l/40 (2.50%) 5.0:.4 1.66:.14 l/60(1.67%) 1.5:.5 0.485116 l/l80 (0.556%) 4. Utility Some of the bodies ofthis invention are extremely dense, impact resistant, wear resistant,extremely hard, and are very strong. They are therefore suitable for usein the numerous ways in which such refractory materials areconventionally used. Some of the other uses to which the bodies of thisinvention can be put include cutting tools, drilling bits, as binders ormatrices for other hard abrasives, and many other specific uses apparentto those skilled in the art.

Bodies of this invention are used in tools in which unusual strength isrequired in combination with high hardness. They are particularlyadvantageous in tools in which conventional cobalt-bonded tungstencarbide tools fail by flaking, chipping, or cracking, such as in toolsfor form cutting, cut-off, milling, broaching and grooving. Thus theyfind extensive use where, because of the inadequacies of cobalt-bondedtungsten carbide of the prior art, high speed steel tools are stillemployed.

Because of the unusual fine grain size, compositions of this inventionare useful in tools where extremely small cross-sections areencountered, as for example in rotary tools smaller than an eighth of aninch in diameter such as end mills, drills and routers; knives having acutting edge with an included angle less than about 30; andsteel-cutting tools which cut with high rake angles such as broaches,thread chasers, shaving or planing tools, rotary drills, end mills, andteeth for rotary saws. While the compositions of this inventioncontaining more than about 12% cobalt are not stronger than compositionsof this invention containing from to 12% cobalt, nevertheless, theimpact strength and toughness is higher. These are generally usefulwhere tool steels are normally employed, and have the advantage ofhigher hardness than tool steels. For highest impact strength,compositions containing from 12 to 25% cobalt are employed, as in somedies and punches. However where a balance of impact strength and wearresistance is required, compositions containing 5 to 12% cobalt alsofind uses in dies and punches employed in operations involving highvolumes and long production runs.

The products of this invention are further illustrated in the followingexamples wherein parts and percentages are by weight unless otherwisenoted.

EXAMPLE 1 This is an example of a composition of this invention in whicha heterogeneous distribution of tungsten in-the cobalt binder phase isproduced by hot pressing a powder of very finely divided tungstencarbide and cobalt containinga very small amount of uniformlydistributed, finely divided free carbon. The tungsten carbide employedis made as described in copending application, Ser. No. 772,810 filedNov. 1, 1968.

By analysis this powder contains 93.4% tungsten, 5.95% total carbon,0.14% free carbon and 0.46% oxygen. Thus there is 5.81% carbon bound inthe tungsten carbide and the atomic ratio of chemically combined carbonto tungsten is 0.95.

The product gives the X-ray diffraction pattern of tungsten carbide andfrom the broadening of the X-ray lines, the average crystallite size iscalculated to be 35 millimicrons. The specific surface area is 6.6square meters/gram. Electron microscopic examination of the powder showsit to consist of porous aggregates of colloidal crystallites in the sizerange 20 to 50 millimicrons. The aggregates are mainly in the size rangeof from 1 to 10 microns, although some aggregates as large as 50 micronscan be observed.

This material will hereafter be referred to as aggregated col-. loidaltungsten carbide powder.

Incorporation of the cobalt bonding phaseis accomplished by milling thecobalt in powder form with aggregated colloidal tungsten carbide powderprepared as described above. To an 8 inch diameter one gallon steel millthe following are charged: (a) 14,000 parts of Carboloy grade 883 cobaltbonded tungsten carbide cylinders, one-quarter of an inch in diameter,and one quarter inch long, the rods being previously conditioned bytumbling for 2 weeks; (b) 1500 parts of the aggregated colloidaltungsten carbide powder prepared above; (c) 205 parts of a fine cobaltpowder, having a specific surface area of 0.7 square meters per gram anda grain size of about 1 micron. This charge occupies about half thevolume of the mill. Milling under acetone is continued for 7 days byrotating the mill at 45 revolutions per minute, after which time themill lid is replaced by a discharge cover and the contents aretransferred to a container maintaining an atmosphere of nitrogenthroughout the system while this is being done. Three portions ofacetone of 395 parts each are used to wash out the mill. The solids inthe drying flask are allowed to settle and the bulk of the acetone issiphoned OK. The flask is then evacuated and when the bulk of theacetone is evaporated, the temperature of the flask is brought to C,maintaining a vacuum of less than a tenth of millimeter of mercury.After about 4 hours, the flask is cooled, filled with pure argon andtransferred to an argon glove box. In this inert environment the solidsare removed from the drying flask and screened through a 70 mesh sieve.

The screened powder is charged to shallow trays which are then loadeddirectly from the argon filled box to a five inch diameter Inconel tubefurnace, where the powder is brought to 900C. at a uniform rate in about3 hours. The gas passing through the furnace consists of hydrogen, at aflow-rate of four liters per minute, with methane introduced at aflow-rate of 40 milliliters per minute. This treatment removes oxygenimpurities, adjusts the carbon content and makes the powder lesssusceptible to reaction with air. The powder is held in this gas streamat 900C. for 2 hours, then is cooled and passed through a 40 mesh perinch screen in an argon filled box. Samples are taken under argon foranalysis.

The resulting heat-treated tungsten carbide powder is characterized byanalysis as follows: tungsten 82.3%, total carbon 5.21%; free carbon0.01%; cobalt 12.1%; oxygen 0.27%. The carbon content found by analysiscorresponds to an atomic weight of combined carbon of 0.965 per atomicweight of tungsten. The free carbon is uniformly distributed throughoutthe powder as particles generally less than a micron in size.

Forty-five parts of the powder described above is charged in anoxygen-free environment to a cylindrical carbon mold and close-fittingcarbon pistons are inserted in each end. The mold containing the powderis pressed at 200 psi and is then transferred to a vacuum hot press.After evacuation the sample, under no pressure, is brought to 1420C. byinduction heating in 7 minutes and held at this temperature with noapplication of pressure for minutes. During the heating the samplesinters and shrinks away from contact with the carbon surface, thusavoiding carburization.

Hydraulic pressure is then applied to both pistons and the pressure onthe sample in the mold is brought to 4000 psi in a period of half aminute. The sample is subjected to a pressure of 4000 psi at 1420C. for1 minute at which time no further movement of the pistons is observed.The mold containing the sample is then ejected from the hot zone andallowed to cool to 800C. in 2 minutes in the evacuated chamber of thepress. After cooling to less than 100C., the mold is removed from thevacuum chamber and a dense sample in the form of a cylindrical disc orbillet, 1 inch in diameter and a quarter of an inch thick, is recovered.

The disc is cut into two segments, using a one hundred and eighty gritdiamond saw, and one of the segments is further cut into bars 0.070 X0.070 inches in cross-section for measurement of strength and hardness.The modulus of rupture of the hot pressed composition as measured byapplying a load at the center of a span of 0.5 inches, is 566,000 psi,the Rockwell A hardness is 91.8. The density of the hot pressed body ismeasured as 14.60 grams per cubic centimeter which corresponds to acomposition containing 9.5% of cobalt. The body contains no free carbon,indicating that the carbon particles have dissolved and reacted duringhot pressing. The reduction in cobalt content as compared with thepowder is due to the extrusion of some metal during fabrication.

The cobalt phase contains tungsten heterogeneously distributed, thetungsten content being about 17% and 7% in different regions asdetermined by procedure A, described above. Two different specimens ofcobalt recovered from different portions of the interior of the billetare scanned by X- ray diffraction repeatedly at one-eighth of a degreeper minute in the region of the strongest cobalt line. That cobalt lineshows a main peak and a shoulder, and gives lattice constants inangstroms, as follows:

First sample: from peaks: 3.5650, 3.5646; from shoulders 3.5520, 3.5515.Second sample: from peaks 3.5646, 3.5643, 3.5646; from shoulders 3.5527,3.5527, 3.5558, 3.5552. Average for peaks 3.565; average for shoulders3.553. These values correspond to about 17 and 7% of tungsten in solidsolution in the cobalt. The fact that both of these values are obtainedon different samples from different parts of the billet tends to provethat the regions high and low in cobalt, respectively, are uniformlyinterspersed in the same way throughout the billet. Furthermore, theregions low in tungsten are apparently derived from the free carbonparticles originally present, which dissolve and combine with thetungsten in the surrounding cobalt, forming tungsten carbide and leavingcobalt regions low in tungsten.

EXAMPLE 2 This is an example of a product of this invention made byblending a tungsten carbide which is deficient in carbon, with graphitepowder and cobalt so that in regions surrounding the graphite, whichmostly dissolves during hot pressing, the cobalt binder is low indissolved tungsten while else where the carbon deficient tungstencarbide furnishes tungsten in solid solution in the cobalt binder.

Very finely divided tungsten carbide having a specific surface area of7.1 square meters per gram is employed. It contains 5.77% total carbonand 0.06 free carbon, and thus the atomic ratio of combined carbon totungsten is 0.93. It contains 1.41% oxygen. Three thousand six hundredparts of this tungsten carbide are mixed with parts of powdered graphitewhich has passed a screen of 200 meshes per inch, and 500 parts of finecobalt powder, and milled as in Example 1 for 5 days. The milled powderis dried, screened and reduced as in Example 1. The resulting powdercontains 5.34% of total carbon, 0.02% of free carbon and 0.26% oxygen.Thus the atomic ratio of carbon to tungsten is 0.99, with the freecarbon evenly distributed through the powder as particles of graphite ofaround :1 micron in size. This powder is hot pressed in a graphite moldas a billet 1.8 inches wide, 3.12 inches long and 0.6 inches inthickness.

The powder is compacted by pushing in the pistons at room temperaturewith a pressure of 400 pounds/square inch. The compact is then heated inthe mold with no pressure applied for a period of 16 minutes, at whichtime the sample has reached 1390C. A pressure of 4,000 pounds/squareinch is then applied for 1 minute and the mold is then removed from theheated zone.

The resulting dense cobalt-bonded tungsten carbide composition is a veryfine-grained, hard, strong material suitable for use as a cutting edgeon tools used for high-speed cutting of ferrous alloys, especially intools such as drills, reamers and form tools where conventionalmetal-bonded carbides are not strong enough and where tool steels areemployed only at low cutting speeds. This use is made possible by thefact that the transverse rupture strength of this product is 540,000pounds per square inch, which is almost twice that of conventionalcarbide tooling, and approaches that of tool steels, while the hardnessis over 91, Rockwell A scale.

Examination of the microstructure shows that there are carbon particlesat the polished surface 10 to 20 microns apart, on the average. Theseare from about 2 to less than 1 micron in size. These carbon particlesare thus within a matrix of cobalt-bonded tungsten carbide in whichthere is an overall carbon deficiency. However, X-ray diffractionmeasurements, by procedure B described above, on the recovered cobaltphase show that some of the cobalt contains less than 8% tungsten insolid solution, whereas the remaining cobalt contains more than 8%tungsten. A low-tungsten region surrounds each grain of carbon. This isevidenced by the fact that near the carbon grains the tungsten carbidegrains are larger than elsewhere, indicating there is no carbondeficiency in those regions and thus little tungsten in the cobalt inthose regions.

EXAMPLE 3 This example describes the preparation of a dense body oftungsten carbide bonded with 12% cobalt possessing unusual high strengthand hardness, having an extremely fine grain size and low porosity, andhaving a heterogeneous distribution of tungsten in the cobalt, made bypreparing a very finely divided intimate mixture of cobalt and tungstencarbide powders, cold pressing and sintering under conditions to bedescribed.

To a steel mill having a capacity of about one gallon and a diameter of8 inches, are charged 14,000 parts of grinding cylinders one-fourth inchlong and one-fourth inch in diameter of tungsten carbide bonded with 6%cobalt. The cylinders have been previously conditioned by tumbling inacetone in the mill for 2 weeks in order to wear off all sharp corners.This pre-conditioning is continued until the rate of wear under millingconditions is less than about 10 parts in 5 days when used to millcompositions of this invention.

Into the mill is also charged 1800 parts of fine commercial tungstencarbide powder, 2 parts of powdered graphite passed through a screen of200 meshes per inch and 1450 parts of acetone. The fine tungsten carbidepowder has a specific surface area as determined by nitrogen adsorptionof 0.66 square meters per gram. By X-ray line broadening the averagecrystallite size is 370 millimicrons. Examination of the powder with anelectron microscope reveals dense aggregates in the size of 2 to 10microns, the aggregates being comprised of tungsten carbide grains inthe size range from 0.5 to 2 microns, with an average of around k or 1micron. Chemical analysis of this powder is 93.2% tungsten, 5.90% totalcarbon, and 0.31% ofoxygen.

The charge occupies about half of the volume of the mill. Milling iscarried out by rotating the mill at 45 revolutions per minute, the lidbeing tightly sealed to prevent loss of contents. Milling is continuedfor 48 hours. The mill is then permitted to cool and is opened. Twohundred and fifty parts of cobalt powder are added. The cobalt powderhas a specific surface area of 0.7 square meters per gram and an averagegrain size of about 1 micron. The mill is closed and milling continuedfor 72 hours, at a rate of 45 rpm. The mill is then permitted to cooland the lid is replaced by a discharge cover and fitted with inlet andoutlet connections so that the contents are transferred to a containermaintained in an atmosphere of nitrogen throughout the operation. Threeportions of acetone of 395 parts each are used to wash out the mill. Thesolids in the receiver flask are allowed to settle and the bulk of theacetone is siphoned off. The flask is then evacuated and warmed from theexterior to distill off the acetone and the temperature of the flask isbrought to 125C. after the distillation is completed. The contents aremaintained at that temperature under a vacuum of less than a tenth of amilliliter of mercury for about 4 hours. The flask is then cooled andfilled with pure nitrogen and transferred to a nitrogen filled glovebox. In this inert environment the solids are removed from the flask andscreened through a sieve having 70 meshes per inch to give essentiallyspherical pellets.

Analysis of the powder, which is maintained continuously under nitrogenis 5.15% total carbon, 0.09% free carbon, 0.46% oxygen, 12.76% cobaltand the remainder being tungsten. The specific surface area by nitrogenadsorption is 2.8 square meters per gram and the crystallite size of thetungsten carbide by X-ray diffraction is 80 millimicrons. The density ofthis powder when tapped in a container to maximum settling is 35 percentof the theoretical density.

This powder has an atomic ratio of combined carbon to tungsten of 0.97and the free carbon is uniformly distributed throughout the powder asparticles less than a micron in size.

Fifty-five parts of the powder described above is charged in anoxygen-free environment to a 1 inch diameter cylindrical graphite moldand close-fittin g graphite pistons are inserted in each end. The moldcontaining the powder is pressed at 200 psi and is then transferred to avacuum hot press. After evacuation the sample, under no pressure, isbrought to 1400C. by induction heating in 7 minutes and held at thistemperature for 5 minutes. During the heating the sample sinters andshrinks away from contact with the mold surface, thus avoidingcarburization.

Hydraulic pressure is then applied to both pistons and the pressure onthe sample in the mold is brought to 4000 psi in a period of half aminute. The sample is subjected to a pressure of 4000 psi at 1400C. for1 minute, by which time no further movement of the pistons is observed.The mold containing the sample is then ejected from the hot zone andallowed to cool to 800C. in 2 minutes in the evacuated chamber of thepress. After allowing to cool to less than 100C. the mold is removedfrom the vacuum chamber and a dense sample in the form of a cylindricaldisc or billet, 1 inch in diameter and a quarter of an inch thick isrecovered.

The resulting hot pressed billet has a transverse rupture strength of545,000 psi and a hardness of 91.4 Rockwell A. Examination of themicrostructure shows extremely low porosity, with an ASTM rating of Al.The cobalt distribution is extremely uniform, the tungsten carbidegrains are substantially all smaller than 1 microns, are generallyequiaxed, no eta phase is observed, and the mean grain diameter is 0.5micron. The carbon content is 5.30 and the atomic ratio of carbon totungsten is 0.96.

The cobalt recovered after removing the tungsten carbide contains 24percent of tungsten, as determined by X-ray diffraction according toprocedure A. The distribution of tungsten in the cobalt phase rangesfrom 7% to 29% with most at about 24% as measured by procedure B.

EXAMPLE 4 This is an example of a product of this invention prepared bythe methods described in Example 1, except that the reduced powder whichis hot pressed contains even more free carbon and the overall level oftungsten in the cobalt is less.

The tungsten carbide is similar to that employed in Example 1 exceptthat it has an atomic ratio of combined carbon to tungsten of 1.0 andcontains 0.26% by weight of free carbon. After milling with cobalt andreduction, it contains 0.07% free carbon and the atomic ratio ofcombined carbon to tungsten is 0.96; the cobalt content is 12.2 percentby weight. After being hot pressed, the dense product has a transverserupture strength of 547,000 psi, a hardness of Rockwell A 92.4, andcontains 9.03 percent of cobalt and an atomic ratio of carbon totungsten of 0.958; no free carbon is found. The acid resistance is lowabout 15 hours. The distribution of tungsten dissolved in the cobaltphase is heterogeneous, regions being identified by procedure B ascontaining 26%, 23.9%, 21.5%, 17.2%, 11.4%, 7.5%, 5.0% and 1.5%oftungsten. No eta phase is present in spite of the relatively lowaverage carbon content. By procedure A, regions averaging 22% and 6%tungsten in the cobalt phase are evident, but procedure B shows thevalue for the regions that gives these averages. Curve D of FIG. Ireflects the analysis by procedure A, and sample 184A of FIG. 4 andTable l reflect the analysis by procedure B.

The billet is cut into cutting tool inserts for turning a hightemperature alloy. Less chipping of the cutting edge is encountered thanwith standard carbide compositions of the prior art.

EXAMPLE 5 This is an example of the invention in which heterogeneousdistribution of tungsten in the cobalt phase is effected by blending twolots of reduced tungsten carbide-cobalt powder, one containing more andthe other less carbon than required to furnish consolidated bodies ofsuperior strength. The reduced powders are prepared as in Example 1, thefirst powder is made from tungsten carbide containing 5.23 percent oftotal carbon, 0.06 percent free carbon and 1.18 percent oxygen, thushaving an atomic ratio of bound carbon to tungsten of 0.85; the secondpowder is made from tungsten carbide containing 6.70% total carbon,0.79% free carbon and 0.51% oxygen. Each powder contains 12.2% ofcobalt. During the screening of the powders through a screen of 70meshes per inch, the horizontal screen and attached receiving pan arevibrated in a direction parallel to the plane of the screen. Theresulting screened powders are obtained in the form of spheres about 50to microns in size formed by aggregation of the much finer powdercomponents. During the reduction step at 900 0, these spheres areslightly sintered and increase in strength so they can be tumbled in amixer without breaking apart.

The first powder after reduction contains 4.54% total carbon, no freecarbon and has an atomic ratio of carbon to tungsten of 0.85. When thispowder is separately hot pressed as in Example 1, the resulting billetcontains 10.96% cobalt and has an atomic ratio of carbon to tungsten of0.83, a Rockwell A hardness of 91.9 and a transverse bending strength ofonly 404,000 psi.

The second powder after reduction contains 5.53 percent total carbon,0.14% free carbon, and an atomic ratio of carbon to tungsten of 1.03.When separately hot pressed as in Example 1, it gives a billetcontaining 8.2 percent cobalt, 5.73 present total carbon, and an atomicratio of total carbon to tungsten of 1.02, a small amount of free carbonbeing present. The hardness is 92.0 on the Rockwell A scale and thetransverse rupture strength is 375,000 psi.

To prepare a composition of this invention 25 parts by weight of thefirst reduced powder and 75% of the second reduced powder are thoroughlyblended by tumbling. This mixture is hot pressed as in Example 1 andgives a billet con-

1. MIXED A CARBON-RICH AND A CARBON-DEFICIENT POWDER TOGETHER PRIOR TOCONSOLIDATION TO PRODUCE A NON-HOMOGENEOUS BINDER;
 2. ADDED FREE CARBONTO A CARBON-DEFICIENT POWDER TO PRODUCE LOCAL AREAS WHERE TUNGSTEN ISREMOVED FROM THE BINDER ALLOY AS TUNGSTEN CARBIDE; OR
 2. A dense body ofclaim 1 bonded with from 8 to 12% by weight of heterogeneouscobalt-tungsten alloy.
 3. A dense body of claim 1 in which the tungstencarbide is present as grains having an average diameter of less than 2microns.
 3. ALLOWED A PORTION OF THE CARBON IN THE TUNGSTEN CARBIDE TOOXIDIZE DURING CONSOLIDATION TO PRODUCE AREAS IN THE BINDER PHASE WHICHARE THEN CARBON DEFICIENT AND HIGH IN TUNGSTEN.
 4. A dense body of claim1 in which the tungsten carbide is present as grains having an averagediameter of less than 1 micron.
 5. A dense body of claim 2 in which thetungsten carbide is present as grains having an average diameter of lessthan 2 microns.
 6. A dense body of claim 2 in which the tungsten carbideis present as grains having an average diameter of less than 1 micron,and the body has a density in excess of 99% of its theoretical density.7. A tool for shaping metal having a cutting edge of a composition ofclaim
 1. 8. A tool for shaping metal having a cutting edge of acomposition of claim 6.